Method of making capacitors containing stannic terephthalate

ABSTRACT

Stannic terephthalate has been prepared and has been found to have an unusually high dielectric constant, electrical and thermal stabilities, and high volume resistivity that make this compound ideal for use as a capacitor dielectric.

This is a continuation of application Ser. No. 07/357,612 filed May 24,1989now U.S. Pat. No. 4,963,127, issued Oct. 16, 1990, which in turn isa continuation of application Ser. No. 07/226,318, filed July 27, 1988,abandoned.

BACKGROUND OF THE INVENTION

Numerous metal salts of terephthalic acid have been reported in theliterature with a broad range cf utilities, most often as additives topolymeric compositions. For example, in U.S. Pat. No. 3,314,919terephthalates of calcium, barium, manganese, zinc and cadmium werereportedly prepared and incorporated into textile fibers for theimprovement of mechanical properties of such fibers. Other examples ofpatent literature disclosing metal terephthalates, principally inpolymeric compositions, include U.S. Pat. Nos. 3,674,894; 3,884,825;3,973,982; 4,039,515; 4,096,109; 4,101,523; 4,198,458 and others. Otherliterature has reported upon metal terephthalates and study of theirvarious properties, but delineation of details of such studies are notconsidered to be relevant to the subject matter of this invention.

SUMMARY OF THE INVENTION

This invention is directed to a new metal salt of terephthalic acid,specifically stannic terephthalate. Stannic terephthalate has beenprepared and it has been found that this compound has an unusually highdielectric constant. Moreover, it has been found that its electricalproperty is highly stable with regard to applied voltage and incidenttemperature. The combination of high dielectric constant, electrical andthermal stabilities, and high volume resistivity make this compoundideal for use as a capacitor dielectric.

DETAILED DESCRIPTION A. Preparation of Stannic Terephthalate

A solution was prepared containing 383 grams of 50% sodium hydroxide inseven liters of water. To this solution was added 400 grams ofterephthalic acid thereby producing a solution of sodium terephthalate.A solution of stannic chloride was prepared by adding 342 grams ofstannic chloride to 0.4 liter water. The stannic chloride solution wasthen added to the sodium terephthalate solution at about 50° C. withcontinuous agitation, causing the development of a white precipitate.The precipitate was filtered, washed with water and dried. The stannicterephthalate produced by the above method was analyzed at 26.8% Sn;42.5% C; 1.65% H. This analysis is within the experimental error ofstannic terephthalate, i.e., Sn(C₈ H₄ O₄)₂ formula which theoreticallyprovides 26.7% Sn, 42.8% C and 1.8% H. Thermogravimetric analysis forthe stannic terephthalate indicated decomposition at 360° C. to aresidue of SnO₂. The infrared spectrum of this salt, obtained in a KBrpellet showed strong carbon-oxygen stretching absorption at 1685 and1290/cm, typical of metal carboxylates having the M+(C═O )O--structure.

Samples of the prepared stannic terephthalate were compacted into flatplates of 10 mils in thickness and 3.5 cm radius at room temperaturebetween polished steel plates at 20,000 psi in a laboratory press.Volume resistivity was determined using an Associated Research, inc.Model 2850 megohm bridge. Dielectric constant was determined using aSprague Electric Model 16 capacitance bridge. The stannic (IV)terephthalate yielded a volume resistivity of 10¹² ohm-cm andsurprisingly had a dielectric constant in the range of 250. The exactvalue of the dielectric constant will vary with sample preparation andcrystal form as it should be understood when the dielectric constantbecomes large. In practice, therefore, the dielectric constant of thissalt may vary from about 50 to several hundred or perhaps higher. Thevalue obtained by the above technique was unchanged when the voltageapplied across the sample was varied from 0 to 500 volts DC. Cycling to300° C. led to a 10% increase in dielectric constant at 300° C. withcomplete recovery on cooling, thereby evidencing no hysteresis.

For comparative purposes, typical metal 5 terephthalates having highvolume resistivities of 10¹² to 10¹⁴ ohm-cm and dielectric constants ofbetween 4 and 5 were tested. These values are given by Ca, Zn, Mg, Al,In, Pb, Cd, Sn(II), Fe(III), Sr, Hg, Co, Ni and Cu(II) terephthalates.Wherefore, it is surprising that stannic terephthalate has an unusuallyhigh dielectric constant and more so that this constant is highly stablewith regard to applied voltages and incident temperature. Thecombination of high dielectric constant, electrical and thermalstability, and high volume resistivity make this compound ideal for useas a capacitor dielectric.

What is claimed is:
 1. In a method of making a capacitor, the improvement comprising incorporating stannic terephthalate as a dielectric material.
 2. In a capacitor structure, the improvement comprising stannic terephthalate as a dielectric material. 